It is common in aircraft to employ automatic flight control systems responsive to inertial sensors, airspeed sensors and the like so as to cause the aircraft to fly in a desired manner. The deviation between a signal indicative of the related control parameter (such as pitch attitude or roll attitude) and a reference signal indicative of the desired parameter (such as desired aircraft pitch attitude, or wings level) provides a related error signal utilized to generate commands to the control surfaces of the aircraft that will correct the situation. The principal manner of establishing a desired reference signal for a given flight parameter is to manually control the aircraft until it exhibits the desired parameter (desired pitch attitude or speed, desired roll angle) while operating a trim release system which causes the reference signal to synchronize to the current actual value of the parameter. Upon removel of trim release, the reference value will be retained as that actual value at the moment of trim release. To further, slightly adjust the reference value (trim up), it is common to use a "beeper" system which typically comprises a switch that the pilot can close to indicate a change in a desired direction of a selected parameter (such as pushing forward for increased pitch attitude). While the switch is thus closed, the reference value (typically held in an integrator) is slewed slowly in the desired direction while the pilot observes the effect which it has. Upon release of the beeper switch, the reference value will remain as adjusted.
There are many situations where the pilot prefers to use a beeper to adjust aircraft trim, but the aircraft cannot respond adequately to the error signal indicative of the difference between the actual aircraft parameter and the reference signal. For instance, during takeoff of a helicopter, if pitch attitude is increased so as to gain airspeed, but spurious ground effects or other aerodynamic influences preclude the aircraft from responding to the command generated by an excessive error signal. Should the pilot continue to beep the system while the aircraft is not able to respond, a very large error signal can build up. The same problem can arise whenever, due to rapid changes in an aircraft parameter (such as pitch attitude), the rate signal which opposes the change may saturate the automatic flight control system (either by virtue of circuit saturation, by virtue of circuit command magnitude limiters, or by virtue of utilization of limited response actuators, which are only slowly recentered). Whenever the automatic flight control system is saturated, continuous beeping by the pilot can result in a very large error. Once the aircraft begins to respond to the large error, the pilot cannot normally beep it back sufficiently fast to overcome the problem. The foregoing problems are further compounded by the fact that actual aircraft attitude lags attitude command signals to some degree, and when airspeed is being controlled by beeping of pitch attitude, there is an additional lag between the achievement of any given attitude and the resulting speed.